Recovery of solid selectively constituted high purity aluminum chloride from hot gaseous effluent

ABSTRACT

RECOVERY OF SELECTIVELY CONSTITUTED HIGH PURITY ALUMINUM CHLORIDE FROM THE GASEOUS EFFLUENT OF THE CHLORINATION OF SODIUM CONTAMINATED ALUMINA INCLUDING THE STEPS OF PURIFYING SUCH GASEOUS EFFLUENT TO PROVIDE ESSENTIALLY CONTAMINANT-FREE ALUMINA CHLORIDE IN GASEOUS FORM IN A GASEOUS CARRIER THROUGH SELECTIVELY COOLING THE HOT GASEOUS EFFLUENT TO A FIRST TEMPERATURE RANGE WELL ABOVE THE AMBIENT CONDITION CONDENSATION TEMPERATURE OF ALUMINUM CHLORIDE YET SUFFICIENT TO CONDENSE A SELECTIVE PORTION OF THE CONDENSABLE CONSTITUENTS THEREIN INCLUDING A SUBSTANTIAL PORTION OF THE SODIUM ALUMINUM CHLORIDE VALUES THEREIN WITHIN CONDENSING APPRECIABLE AMOUNTS OF ALUMINUM CHLORIDE, INITIALLY SEPARATING FROM THE COOLED GASEOUS EFFLUENT ENTRAINED PARTICLES AND INITIALLY CONDENSED CONSTITUENTS, FURTHER REDUCING THE TEMPERATURE OF THE RESIDUAL COOLED GASEOUS EFFLUENT FROM THE FIRST STAGE SEPARATION TO A SECOND PREDETERMINED LOWER TEMPERATURE RANGE STILL ABOVE THE CONDENSATION TEMPERATURE OF ALUMINUM CHLORIDE TO EFFECT A FURTHER SELECTIVE CONDENSATION AND SEPARATION OF THE REMAINING CONDENSABLE CONSTITUENTS THEREIN, AND DIRECTLY DESUBLIMING THE ESSENTIALLY CONTAMINANT-FREE GASEOUS ALUMINA CHLORIDE VALUES FROM THE PURIFIED RESIDUAL GASEOUS EFFLUENT TO SELECTIVELY CONSTITUTED SOLID FROM IN A FLUIDIZED BED OF SOLID PARTICLES OF ALUMINUM CHLORIDE AT CONTROLLED TEMPERATURES.

L. K. KING ET AL 3,786,135

Jan. 15, 1974 RECUVIJRY 01' 501410 SELBCTLVELY ULINSTLTUTED HLGHPURE'I'Y ALUMINUM CHLORIDE FROM HOT GASEOUS EFFLUENT 3 Sheets-$heet 1Filed Sept. 14, 1971 SCRUBBER 'COQLANT CHLORINATOR INVENTOR5 LAKE) K.KIA/G LESTEQ L. K/VAPF ,qaA/Aw C; .501054'5? IV/(HOLAS K1 OAP BERNARD m.STARNER JOHN A. REMPER Jan. 15, 174 KING ETAL 3,786,135

RECOVERY OF SOLID SELECTIVELY CONSTITUTED HIGH PURITY ALUMINUM CHLORIDEFROM HOT GASEOUS EFFLUENT Filed Sept. 14, 1971 3 Sheets -Sheet z Jam 15,1974 K. KING ETAL 3,786,135

RECOVERY OF SQLID SELECTIVELY CONSTITUTED HIGH PURI'IY ALUMINUM CHLORIDEFROM HOT GASEOUS EFFLUEN'J 5 Sheets-Sheet 3 Filed Sept. 14, 1971 UnitedStates Patent Filed Sept. 14, 19711, Ser. No. 180,276 Int. Cl. C01f7/58, 7/60, 7/61 US. Cl. 423-496 9 Claims ABSTRACT UF THE DISCLOSURERecovery of selectively constituted high purity aluminum chloride fromthe gaseous efiluent of the chlorination of sodium contaminated aluminaincluding the steps of purifying such gaseous efiluent to provideessentially contaminant-free aluminum chloride in gaseous form in agaseous carrier through selectively cooling the hot gaseous efiluent toa first temperature range well above the ambient condition condensationtemperature of aluminum chloride yet sufficient to condense a selectiveportion of the condensable constituents therein including a substantialportion of the sodium aluminum chloride values therein withoutcondensing appreciable amounts of aluminum chloride, initiallyseparating from the cooled gaseous efliuent entrained particles andinitially condensed constituents, further reducing the temperature ofthe residual cooled gaseous eflluent from the first stage separation toa second predetermined lower temperature range still above thecondensation temperature of aluminum chloride to effect a furtherselective condensation and separation of the remaining condensableconstituents therein, and directly desubliming the essentiallycontaminant-free gaseous aluminum chloride values from the purifiedresidual gaseous effluent to selectively constituted solid form in afluidized bed of solid particles of aluminum chloride at controlledtemperatures.

This invention relates to the production of aluminum chloride andparticularly to a process for treatment of the hot gaseous effluent fromthe chlorination of alumina for the quantity production of selectivelyconstituted high purity solid aluminum chloride.

Although the potential advantages of utilizing aluminum chloride as asource material in the electrolytic production of metallic aluminum havelong been recognized, commercial realization thereof has been precludedby the inability of the art to provide aluminum chloride of sufficientlyhigh purity and of a character as to be utilizable therein and toprovide aluminum chloride in any significant required quantity thereforin an economically acceptable manner. The long standing incentive andneed for economically producible high purity aluminum chloride hasresulted in extensive experimental exploration and evaluation ofnumerous suggested expedients for obtaining such long desired result.However, to date none of these suggested expedients has succeeded insatisfying the long desired objective of commercial quantity productionof economically producible high purity aluminum chloride.

In general, the reduction of alumina-containing materials with chlorinein the presence of reducing carbon in some form to produce aluminumchloride is an old and well-known reaction and one of the suggestedexpedients referred to above utilized bauxite as the alumina-containingmaterial. Such reaction proceeds vigorously and usually results in theprovision of aluminum chloride in gaseous form in the elevatedtemperature gaseous eilluent thereof. Bauxite, however, normallycontains many impurities including iron oxide, silica and titania. Sincethese impurities readily react with chlorine in the presence of carbonto form iron, silicon and titanium chlorides, the separation andrecovery of aluminum chloride values from the hot reaction eflluent fromthe chlorination of bauxite has posed particularly diflicult problemsbecause of the presence of multiple impurities therein and because ofthe inherent characteristics of aluminum chloride during separationoperations, and especially because of the influence of the vaporpressure of aluminum chloride on the degree of condensation thereofunder ambient conditions.

Likewise, although moisture or other forms of hydrogen are often presenton the carbon intermixed bauxite used for such chlorination reaction,this has not been heretofore considered detrimental since such hydrogenis converted to hydrogen chloride which can react with the ironimpurities present. Inasmuch as such gaseous aluminum chloride reactionmixture necessarily required after-purifi cation, the presence of suchby-products in the reaction mixture was not only not of great concern,but the use of formed hydrogen chloride to reduce iron impuritiespresent in the bauxite has been relied on as one manner of convertingsuch impurities to a form facilitating their removal.

This invention, however, is particularly directed to aluminum chlorideproduction processes that employ Bayer process alumina as an initialreactant, which, because of its caustic soda treatment, is normallycontaminated with sodium impurities, e.g. soda (Na o), which lead to theformation of sodium aluminum chloride and other sodium based impuritiesduring chlorination thereof.

This invention may be briefly described, in its broad aspects, as animproved, efiicient and economic process for the production ofselectively constituted high purity solid aluminum chloride through theselective separation of condensable and other imprities from the hotgaseous efiluent of the chlorination of sodium contaminated alumina,such as Bayer process alumina that is essentially free from iron,silicon and titanium impurities by reducing the temperature thereof tobelow the chlorination reaction temperature and above the condensationtemperature of aluminum chloride under the ambient conditions andeconomic quantity recovery of selectively constituted high purityaluminum chloride by direct desublimation from the residual gaseouseffiuent thereof. In its narrower a pects, the invention includes thesteps providing an essentially contaminant-free aluminum chloride ingaseous form in a gaseous carrier by initially cooling such eifluent toa first predetermined temperature range of below the chlorinationtemperature and well above the condensation temperature of aluminumchloride under ambient conditions to condense a selected portion of thecondensable constituents therein including odium aluminum chloridevalues and permit the separation therefrom of entrained solid and liquidparticles and said initially condensed volatile constituents includingsodium containing reaction products, preferably followed by a secondarycooling of the res dual gaseous effluent to a second and lowerpredetermined temperature range above the ambient condition condensationtemperature of aluminum chloride to selectively condense the remainingcondensables other than the aluminum chloride values contained thereinand permit the second stage separation therefrom of any remainingentrained particles and further condensed constituents to provide aresidual cooled gaseous eflluent comprising chlorine, phosgene andcarbon monoxides and containing essentially contaminant-free gaseousaluminum chloride as a condensable constituent therein and recoveringsaid aluminum chloride values therefrom as selectively constituted highpurity particulates by direct desublimation in a self-replenishingfluidized bed of particulate aluminum chloride maintained at apredetermined temperature substantially below the upper ambientsolidification temperature of aluminum chloride.

Among the advantages of the subject invention is the permitted efiicientand economic commercial quantity production of selectively sized andcontoured high purity aluminum chloride of a character particularly wellsuited for the production of aluminum metal by the electrolyticreduction thereof.

A primary object of this invention is the provision of an efficient andeconomical process for the continuous commercial quantity production ofselectively sized and contoured high purity aluminum chloride from thegaseous effluent of the chlorination of sodium contaminated alumina.

Another object of this invention is the provision of an improved processfor the economical recovery of high purity aluminum chloride values fromthe gaseous effluent of the chlorination of sodium contaminatedparticles of alumina by preliminary separation therefrom atpredetermined temperature levels of entrained particles, e.g. solids andliquid particles, and condensable volatile constituents includingsodium-containing reaction products, followed by direct desublimation ofhigh purity aluminum chloride from the purified and essentiallycontaminantfree residual gaseous efiluent thereof.

A further object of this invention is the provision of an improvedprocess for recovering high purity aluminum chloride from the gaseousefiiuent from the chlorination of alumina and containing aluminumchloride, carbon oxides, entrained solid and liquid particles includingalumina, carbon, aluminum oxychloride and/or aluminum hydroxychloride aswell as condensable volatile constituents including volatilized sodiumaluminum chloride, by initially cooling such hot gaseous effluent to afirst predetermined temperature level below the chlorination reactiontemperature and above the ambient condition condensation temperature ofaluminum chloride to selectively condense a substantial portion of thecondensable constituents therein including most of the sodium aluminumchloride values therein, further cooling the residual gaseou efiluenttherefrom to a second and lower temperature level to selectivelycondense effectively all of the remaining condensable constituents otherthan aluminum chloride therein, and directly desubliming the aluminumchloride values in a self-replenishing fluidized bed maintained at atemperature level substantially below the solidification temperature ofsuch aluminum chloride, such steps being carried out in the substantialabsence of hydrocarbons, free hydrogen-containing gases, freeoxygen-containing gases and moisture.

It is still another object of the invention to provide an economic andelficient process for the continuous, commercial quantity production ofselectively contoured, readily handleable and fiowable high purity solidaluminum chloride particles of generally lobular contour that areparticularly well suited for utilization in the electrochemicalproduction of metallic aluminum.

A still further object of this invention is the provision ofeconomically producible selectively constituted high purity aluminumchloride product.

A still further object of this invention is the provision of solid,selectively sized, high purity particles of aluminum chloride ofgenerally lobular contour and characterized by the effective absence ofplanar exterior surfaces and relatively sharp protuberant angles.

Other and further objects and advantages of the present invention willbecome apparent from the following portions of this specification andfrom the accompanying drawing which illustrates the principles of thisinvention in conjunction with illustrative appartus utilizable in thepractice thereof and in which:

FIG. 1 schematically delineates suitable arrangements of apparatusutilizable in the practice of the inventive process in accord with the pinciples of this invention- FIGS. 2a, 2b and 2c are photomicrographs, at30, 200 and 500 magnifications respectively, illustrative of theselective configuration of a preferred product obtained through thepractice of this invention.

FIGS. 3a, 3b and 3c are photomicrographs, at 30, 200 and 500magnifications respectively, illustrative of the selective configurationof a smaller sized product obtained through the practice of thisinvention.

The hot gaseous efiluent emanating from the chlorination of sodiumcontaminated Bayer process alumina in the presence of carbon and in thesubstantial absence of hydrocarbons, free hydrogen containing gas, freeoxygen containing gas and moisture will normally contain, in addition togaseous aluminum chloride values, carbon oxides, preferablypredominantly carbon dioxide, entrained particles of both solids andliquids, and condensable volatile constituents including amounts ofvoltailized sodium aluminum chloride values, i.e. correspondingsubstantially to the sodium impurities content of the alumina subjectedto chlorination.

In accordance with the principles of this invention and by way of broadexample, a hot gaseous efiluent of the general type described, afterexiting at 3 from a chlorination vessel 1, is initially cooled in a heatexchanger 5 to a first predetermined temperature range of between about200-600 C., and preferably between 250350 C., which is well below thechlorination reaction temperature and well above the ambient conditioncondensation temperature of aluminum chloride (which is normally about180 C. subject to ambient vapor pressure conditions) and which will beeffective to initially condense a substantial portion of the condensableconstituents therein including a corresponding portion of thevolatilized sodium aluminum chloride values present therein. Suchthereby initially condensed constituents, which will include asubstantial portion of the total sodium aluminum chloride value contentthereof, in the form of a complex mixture with attendant aluminumchloride, together with most of the entrained solid and liquidparticulates will be then separated from the gaseous carrier in a firststage separator 7. Such separated mass can constitute an appreciableproportion, i.e. as much as l5-25% by weight of the average aluminabeing chlorinated. Further cooling of the residual gaseous effluent fromthe first stage separation as by passage thereof through a second heatexchanger 9 reduces the temperature thereof to a second and still lowerpredetermined temperature range of between about -250 C. This secondpredetermined temperature range, which is still above the ambientcondition condensation temperature of aluminum chloride will effect thecondensation of essentially all of the remaining volatile constituentstherein that are condensable above the condensation temperature ofaluminum chloride, i.e. including essentially the remainder of thesodium aluminum chloride as a remaining condensable volatile constituenttherein, and still without significant condensation of the still gaseousaluminum chloride values therein. Following the separation of suchsecond stage condensates and any remaining entrained particulates fromthe now depleted residual effluent gas streams as effected by thedescribed passage thereof through a second stage separator 10, suchresidual and depleted effluent gas stream comprising essentiallycontaminant-free gaseous aluminum chloride is introduced into afluidized bed 17 of particles of aluminum chloride maintained at a thirdpredetermined temperature range well below the ambient conditioncondensation temperature of aluminum chloride and at about 3'0-l00 C.,suitably within about 60-90 C. and preferably within the narrower rangeof 5070 C. to effect the direct desublimation of the aluminum chloridevalues therein to solid form. Desublimation as utilized herein refers tothe direct formation of solid aluminum chloride from the gaseous phasewithout any noticeable formation of an intermediate liquid phase.

The initial and second cooling steps and the concomitant separatingsteps are desirably also carried out in the substantial absence ofmoisture, preferably to the extent that the aluminum chloride epentuallyrecovered will contain a total of less than about 0.3% and especiallyless than about 0.1% by weight of combined oxygen.

As noted above, the more readily condensable constituents or impuritiesin the hot gaseous efiluent recovered from the chlorination zone willinclude sodium aluminum chloride values, usually in combination orcomplex form with a minor attendant amount of aluminum chloride, andentrained particles which include liquid particles such as aluminumoxychloride values and/or aluminum hydroxychloride values; and solidparticles such as alumina, carbon, and intermixtures thereof.Accordingly, a selective yet substantial portion of the sodium aluminumchloride values will be condensed upon the initial cooling and will beseparated in the first stage separator in the form of a complex mixtureof sodium aluminum chloride with aluminum chloride, plus a substantialportion of the aluminum oxychloride and/or aluminum hydroxychloride, andthe above noted solid particles. Essentially the remainder of thecondensable volatile constituents in the residual initally cooledeflluent gas Will normally condense upon the further cooling thereof andsuch sec-' ondarily condensed remainder which may include any remainingcomplex mixture of sodium aluminum chloride with aluminum chloride, andany remaining solid or liquid particles as identified above may then beseparated prior to the recovery of the aluminum chloride from theresidual gaseous efiiuent.

While not completely understood at the present time, it is believed thatthe volatilized sodium aluminum chloride predominantly condenses as aresult of the selective initial cooling of the hot gaseous efiluentrecovered from the chlorination reactor in such a way that under theambient conditions the condensed sodium aluminum chloride, more or lessin the form of a complex with attendant aluminum chloride, as well asthe liquid aluminum oxychloride and/ or aluminum hydroxychloridepresent, readily deposit on the entrained solids and to a great extentsettle out as larger droplets than otherwise for collection even beforereaching the operative physical separation area of the first stageseparator 7. Consequently, instead of depositing as smaller droplets onthe separating media thereof, such condensed volatile liquidconstituents and entrained liquid particles more or less deposit on theentrained solids to form larger droplets which are readily separablewithout unduly burdening the separator apparatus employed.

The remainder of the volatile sodium aluminum chloride and any remainingaluminum oxychloride and/or aluminum hydroxychloride values which arestill present in the residual gaseous effluent from the first stageseparator, are thereafter condensed upon the second stage cooling of thegaseous efiluent.

Such second stage condensed constituents, which will largely be in thenature of a mist or fog of NaAlCh-AlCl and any remaining entrained ordissolved solids such as alumina and carbon dust and liquids such asaluminum oxychloride and/ or aluminum hydroxychloride, may then beseparated from the further cooled gaseous effluent. Such can beefiiciently and continuously removed in a simplified manner by a secondstage separator functioning essentially as a demister, e.g. a finerpored filter means, in an amount corresponding to the remainingimpurities present.

In accordance with the foregoing, it will be realized that the sodiumaluminum chloride values that condense out and which are removed at thefirst and second stages of separation generally condense to liquid formas a complex mixture with an appropriate minor amount of the aluminumchloride present, and such mixture generally contains in associationtherewith or dissolved therein not only concomitantly removed aluminumoxychloride and aluminum hydroxychloride but also traces of chlorine,phosgene, and the like, as well as alumina which stems from entrainedalumina dust originally present or which forms by reaction of aluminumchloride with moisture which may be present, thereby also forminghydrogen chloride.

As noted above, the final recovery of the aluminum chloride from thepurified gaseous efiluent, and which may also contain chlorine,phosgene, carbon monoxide and dioxide, is efiected by single stagedirect desublimation in a fluidized bed 17 of aluminum chloridemaintained at a temperature substantially below the solidificationtemperature of aluminum chloride and to thereby endow the bed with aself-replenishing character as well as providing a readily recoverablesolidified aluminum chloride product of notable high purity and in afine readily handleable and flowable form. More particularly, suchaluminum chloride product will normally be of a purity of at least 99.5%and quantitative, economically producible solid aluminum chloride of apurity in excess of 99.8% is readily obtainable in accord with theprinciples of this invention. Such solid high purity product will alsohave an average particle size distribution of about 40350 mesh (U.S.Sieve Series), and predominantly about 350 mesh which is particularlysuited for immediate utilization in an electrolytic aluminum recoverycell for the efiicient electrochemical formation of pure aluminum metal.

The use of markedly low desublimating or condensing temperatures incomparison to the solidification temperature of aluminum chloride, e.g.30l00 C. as compared to C., subject to the corresponding ambientconditions, and the agitation inherently present in the fluidized bed ofparticles of aluminum chloride surprisingly results in the formation ofa preferred range of size of particles than that developed bydesublimation at substantially higher temperatures in the vicinity ofthe actual ambient condition solidification temperature of aluminumchloride. Since such range of particle sizes contribute to easy handlingfor subsequent electrochemical conversion of the aluminum chloride tometallic aluminum, the use of desublimation temperatures well below theupper ambient limits thereof according to the principles of thisinvention is highly advantageous. Thus, rapidly quenching the gaseousaluminum chloride in the highly agitated fluidized bed from atemperature of about ISO-250 C. to below 100 C., and preferably to about60 C., in a single stage or step one would not have expected thatreadily handled and flowable particles would form.

The final condensation of the aluminum chloride is preferably carriedout in the substantial absence of moisture so that the high purity solidform aluminum chloride so produced contains a total of less than about0.5% by weight of free water and combined reaction products of water andaluminum chloride. Additionally, the condensation is preferably carriedout in the substantial absence of free hydrogen-containing gas, freeoxygen-containing gas, and non-volatile impurities under the ambientconditions sufficiently that the solid form alumina chloride produced bythe condensing contains a total of less than about 0.3% and preferablyless than 0.1% by weight of combined oxygen and non-volatile impurities.

As a result of the cascaded purification and desublimation recoverysteps according to the invention, a highly purified and readily flowablesolid particle aluminum chloride product is obtained having a total ofless than about 0.3% by weight of combined oxygen and nonvolatileimpurities, i.e. which is essentially free from sodium, iron, siliconand titanium impurities, and preferably also having an average particlesize distribution of about 40-350 mesh.

The described desublimation of the aluminum chloride can be carried outat negative or vacuum pressures, e.g.

down to about 0.1 atmosphere absolute, as well as at positive orelevated pressures, up to about 10 atmospheres absolute, subject toconsiderations of partial pressure of the aluminum chloride presentunder the ambient conditions. A preferred total operating pressure isabout 1 atmosphere absolute.

Thus, by careful selection and control of the temperature ofdesublimation, and by using a fluidized bed of aluminum chlorideparticles in the condensation zone, it has been found that the particlesize of the solidified aluminum chloride can be selectively controlled.At lower temperatures within the specified range of about 30- 100 C.,the average particle size of the desublimed product is generally smallerthan those obtained at higher temperatures within such range. Dependingon the particular temperature chosen, the aluminum chloride particlesrecovered will have an average particle size of about 40 to 350 mesh,and predominantly about 100-350 mesh. Operating temperatures of lowerthan about 30 C. for the fluidized bed are generally uneconomic andundesired since excessive cooling water costs are necessitated andexcessive amounts of aluminum chloride fines are condensed out of theeflluent, Whereas temperatures above about 100 C. under the contemplatedoperating conditions result in the undesirable loss of undue amounts ofuncondensed aluminum chloride in the gaseous efiluent.

For example, even at a relatively low temperature of about 90 C., ascompared with an aluminum chloride solidification temperature of about180 C. at 1 atmosphere absolute and of about 150 C. at about 0.5atmosphere absolute, a certain amount of the gaseous aluminum chloridevalues will not desublime since the vapor pressure conditions in thefluidized bed favor retention thereof in the gaseous state thereof. Inthis regard, under the ambient conditions of about .5 atmosphere, thevapor pressure of AlCl is 1 mm. at 100 C., 0.32 mm. at 90 C. and 0.004mm. at 60 C., which confirms the desirability of using condensationtemperatures at the lower end of the stated range of 30-100" C.,especially where lower ambient pressures are employed. Naturally, thepresence of material that complex with the aluminum chloride may modifysomewhat the aluminum chloride partial pressure in the system and thusthe condensation temperature thereof.

Referring again to the drawings which exemplarily illustrate inschematic form certain presently preferred apparatus components suitablefor use in the practice of the invention, the heat exchange coolingassembly 5 may suitably constitute a conventional shell and tube heatexchanger supplied with fluid coolant such as a liquid Dowtherm coolant(Dow Chemical Co. product). Alternatively, the initial cooling of thehot gaseous efiluent could be effected by introduction of dry inert gasor solid particles of aluminum chloride, into direct contact with thehot gaseous efiluent or by other suitable means.

The first stage separator 7 may suitably constitute a vertical filterchamber conventionally including a downstream outlet 7a at the upper endthereof for discharge therefrom of the residual initially cooled gaseouseflluent and smooth imperforate perimetric side walls 7b of heatdissipating character dependently terminating in a collection hopper 70for removal of the separated condensate therefrom. Included within theseparator housing is a plurality of permeable filter members 13,suitably of stone or polished ceramic material.

The filter members 13 and the surrounding environment are kept at asufficiently high enough temperature, e.g. about 200500 C., so as tominimize, if not effectively preclude condensation of the remainingvolatiles in the gaseous effluent.

While operation of the described system will normally effect a furtherreduction in the temperature of the residual effluent gas emanatingtherefrom, such temperature reduction can be supplemented by the passageof the residual efl'luent through the second stage heat exchanger 9 tobring the same within the desired second predetermined temperature levelof about ISO-250 C.

This further cooled residual gaseous efiluent is then introduced intothe second stage separator 10 wherein the further condensed sodiumaluminum chloride-aluminum chloride complex values together with anyremaining entrained or dissolved solids such as coked alumina and carbondust and liquids such as aluminum oxychloride and/or aluminumhydroxychloride, are separated continuously as further impurities fromthe residual and nonfurther cooled aluminum chloride-containing gaseouseflluent.

While the temperature of the gaseous mixture in the second stageseparator 10 is sufliciently low to insure condensation of essentiallythe remainder of the condensable constituents present and especiallysodium aluminum chloride and/ or sodium aluminum chloride-aluminumchloride complex mixture, e.g. about -250 C., substantially no aluminumchloride will condense since the temperature is still above thecondensation temperature of aluminum chloride under the ambientconditions. The gaseous effluent from the second separating stage willthus consist essentially of gaseous aluminum chloride of high puritywhich is essentially free from contaminants such as sodium aluminumchloride, aluminum oxychloride, aluminum hydroxychloride, alumina andcarbon.

The second stage separator 10 may suitably comprise a permeable filtermedium such as a stone filter 15 similar to those utilized in the firststage separator 7.

The above described residual gaseous aluminum chloride-containingeffluent is introduced into the fiuidizing bed chamber at a locationremote from any contact surfaces in the vessel, to prevent undesired orpremature condensation of the gaseous aluminum chloride to liquid orsolid phase at the inlet and at any such contact surfaces withconsequent plugging and insulating deterioration of heat exchangesurfaces therein due to the hard and abrasive resistant character of thesolid aluminum chloride. Likewise, the temperature, vapor pressure andlocus of desublimation must be controlled to avoid premature anddetrimental condensation of the gaseous aluminum chloride at locationsother than that heretofore described to effectively avoid clogging ofthe inlet entrance and deposition on cold surfaces of the chamber.

As illustrated in the drawing, the cooled and purified aluminumchloride-containing residual gas which will be at a temperature that issufficiently high to preclude condensation of the aluminum chloridevalues therefrom under the ambient condition is introduced through afeed line 16 into a fluidized bed of particles of aluminum chloridedisposed within a chamber 4. The fluidized bed chamber 4 includes afiuidizing gas distribution inlet 12 at its lower end supplied withfiuidizing gas through line 11, a separator 21, suitably a permeablefilter medium 23 and a residual gaseous effluent outlet 29 at its upperend connected to an outlet conduit 22, and an aluminum chloride outlet24 at the dependent terminus of inclined perforator distribution plate25 disposed adjacent the bottom of the chamber for withdrawing condensedaluminum chloride solids therefrom. Disposed within the fluidized bed isa heat exchanger such as finned coils 26 for cooling the contentsthereof and for maintaining the temperature of the bed withinpredetermined ranges. The purified residual gas exiting from the secondstage separator 10 and containing the gaseous aluminum chloride valuesis introduced into the fluidized bed at a location remote from thecooling fins and from any contact surfaces therein.

A residual gaseous effluent recycle conduit 27 is connected intermediatethe residual gaseous efiluent outlet conduit 22 and the fiuidizing gasdistribution inlet 20 to permit the recycling and use of portions of theresidual gaseous efiluent as fiuidizing gas. For such purpose, acompressor or pump 19 is desirably interposed in the outlet conduit 22.Alternatively, an independent source of fluidizing gas can be fed viasupply line 28 for fluidizing the bed, instead of or in addition to theresidual efiluent gas fed via recycle conduit 27.

The inlet 30 for the gaseous aluminum chloride-containing gas isdesirably provided with means to maintain the temperature of theincoming gas at an elevated value, such means shown schematically at 31,may suitably comprise auxiliary heating means, such as electricalresistance heating means, or may comprise heat insulation material, suchas quartz, alumina, graphite, asbestos and the like, at the entrancethereof to minimize, if not prevent, premature cooling and liquificationor solidification of the gaseous aluminum chloride passing therethroughwhich would tend to clog the same to impede or otherwise deleteriouslyaifect the desired condensation or desublimation operation.

Because of the need to avoid premature condensation of the gaseousaluminum chloride at locations other than in the fluidized bed itself,considering the ambient conditions, the entrance of inlet 30 desirablyprojects appreciably into the bed and terminates remote from allstructural surfaces therewithin including the walls of the chamber andthe cooling means 26. In this way, as the incoming gaseouschloride-containing carrier enters the condenser chamber 4 in suchmanner as to immediately contact the bed particles the aluminum chloridevalues therein will condense before there is any chance of contactthereof with the adjacent apparatus surfaces. By the time the mixture ofresidual gaseous efiiuent and fiuidizing gas exits from the top of thebed, the aluminum chloride values therein have sufliciently changed tosolid phase and built up in solid particle form to avoid significantentrainment in the exiting gaseous mixture and is heavy and hard enoughto operate as a particle component of the fluidized bed in the vicinityof the chamber walls and cooling means without danger of depositionthereon.

The ambient conditions at the locus of condensation should be such thatthe vapor pressure of the aluminum chloride is just low enough todesublime the same to solid form without causing any deposition ofresidual aluminum chloride either in solid or liquid form on the surfaceof the permeable filter medium 23 of the separator 21.

As previously noted, the herein described method of purification resultsin the provision of a gaseous effiuent consisting essentially ofcontaminant-free aluminum chloride in gaseous form in a gaseous carrierand from which commercially utilizable quantities of solid high purityaluminum chloride may be economically recovered, as for example bycondensation, therefrom. Such recovered aluminum chloride product willnormally be of a purity of at least 99.5% and purities of 99.8% andbetter are readily obtainable in quantity production operations. Morespecifically, practice of the subject invention permits the economicattaining of solid aluminum chloride in quantity production that isessentially free of sodium, iron, silicon and titanium impurities andhaving a total nonvolatile, including combined oxygen, content of lessthan 3%, desirably of less than .1% and preferably of less than .03%.Aluminum chloride having less than .1% of combined oxygen and othernon-volatile content is particularly suited for utilization inelectrolytic reduction cells for production of aluminum and thusmarkedly contributes to a long sought after objective in this art.

The singular nature of the product resulting from the hereinabovedescribed direct desublimation recovery of aluminum chloride values isillustrated, under various magnifications in FIGS. 2 and 3 of thedrawings. As best shown in FIG. 2a (under 30X magnification) theparticles of aluminum chloride are of generally spheroidal characterpresenting a generally lobate curvilinear external contour andcharacterized by a marked absence of planar exterior surfaces andrelatively sharp protuberant angles that are normally characteristic offracture planes or the like. As becomes apparent from iFIGS. 2b and 2cthe particles of aluminum chloride are compositely constituted ofagglomerated, cemented or otherwise autogenously bonded pluralities ofsmaller sized particles of rather widely varying dimension but ofgenerally spheroidate character. Because of such composite constitutionthe exterior surface of the particles, while still curvilinear incharacter, are of generally lobular and bul late character and presentmarked localized departure from true spheroidal character and. hence theterm lobul-ar will be herein utilized to describe the general characterof the resultant particles.

FIG. 3a illustrates (under 30X magification) a much finer aluminumchloride product obtainable by the practice of this invention. Asevidenced by FIGS. 3b and 30 however the particles here are of markedlymore lobate character. It is equally apparent however that the particlesagain present a generally lobular curvilinear external contour and arecharacterized by a marked absence of planar exterior surfaces andrelatively sharp protuberant angles and are compositely constituted ofagglomerated or otherwise joined pluralities of smaller sized particlesof widely varying dimensions but of generally spheroidate or lobatecharacter.

The solid lobular aluminum chloride particles of generally curvilinearcontour of the invention essentially contain rounded lobes of lobuleswhich often impart an apparent bullate, blistered and/or nodularcomposite surface configuration.

As will now be apparent to those skilled in this art the generallylobate character of this product dilfers markedly from conventionallyproduced aluminum chloride that is commercially available. Not only doesthis new material provide marked advantages in both handling andflowability but the herein described invention avoids any crushing orgrinding operation with their attendant contamination with impuritiesfrom the equipment employed but more significantly avoids exposure ofthe aluminum chloride product to the air with its ever attendant hazardof contamination by air borne moisture.

The new product obtained through the practice of this invention has abulk density in the range of about 75 to lbs. per cu. ft. (for aparticle size range of from about 40 to 350 mesh). Samples of FIG. 3type product have been found to have an angle of repose in the range ofabout 35 to about 41, and a mean of about 38, when measured in a drynitrogen atmosphere by the International Standards Organization MethodISO/PC47 (Secretariat 247) 424 for measurement of angle of repose ofalumina. Because of the greater spheroidicity of the FIG. 2 typeproduct, lower angles of repose will normally be characteristic thereof.As a matter of caution, however, it should be noted that the magnitudeof the numerical comparative criteria of this latter characteristicdepends largely upon the measurement techniques employed and there islittle, if any, standardization in such measurement techniquesgenerally, and the requirement of maintaining this particular product ina contaminant-free environment further complicates the problems ofmeasurement thereof.

EXAMPLE The following example is set forth to illustrate withoutlimitation, various features of the present invention.

Gaseous efiluent from the chlorination of coked sodium contaminatedalumina at about 700 C. is fed to an indirect heat exchange coolingconduit 5 fed with Dowtherm coolant in an amount sufficient to cool thegaseous efiiuent to about 250 C. Such gaseous efliuent contains vaporous0r gaseous aluminum chloride as well as carbon dioxide and carbonmonoxide, together with entrained dust including carbon and cokedalumina as well as residual impurities present in the starting material.Specifically, the

1 1 gaseous efiluent has a total of less than 0.3% by weight free oxygenand a total of less than 0.5% by weight free water and combined waterconstituting reaction products of water and aluminum chloride.

The 250 C. cooled gaseous efiluent is then separated from entrained dustand liquids particles and thereby condensed volatiles in the first stageseparator 7. Such entrained dust and liquids particles and condensedvolatile constituents so separated include sodium aluminumchloride-aluminum chloride eutectic mixture, in consequence of sodiumimpurities present in the starting alumina, as well as aluminumoxychloride, aluminum hydroxychloride, alumina and carbon, and traces ofchlorine, hydrogen chloride and phosgene, all of which are continuouslyrecovered more or less in the form of a partially dissolved mass. Thecondensed volatile constituents therein are those which condense out asa result of the cooling of the gaseous efiluent from the chlorinationtemperature to 250 C.

This recover mass can amount to as much as 17% by weight of the averagecontent of the chlorinator bed and about 12% by weight of such mass willconstitute sodium impurities calculated as Na O, i.e. in the form of asodium aluminum chloride-aluminum chloride eutectic mixture.

The residual efiluent gas leaving the first stage separator 7 is thenfurther cooled to about 200 C. and introduced into the second stageseparator 10 wherein the remaining content of sodium aluminumchloride-aluminum chloride complex mixture, aluminum oxychloride andaluminum hydroxychloride, and any residual alumina and carbon dust, andany remaining traces of chlorine, hydrogen chloride and phosgeneoccluded therewith, which meanwhile condense more or less as a mist, areremoved. These further impurities present in the gas, including thecondensables which posses a higher condensation temperature than that ofaluminum chloride under the ambient conditions, are conveniently removedat this point so as to provide a relatively pure final gaseous eflluentin outlet line 16 containing essentially only aluminum chloride, carbondioxide and carbon monoxide as well as trace amounts of otherimpurities, e.g. chlorine, hydrogen chloride, phosgene, carbontetrachloride, and the like, from which the aluminum chloride is readilyrecovered.

Such relatively pure gaseous eflluent at about 200 C. enters the chamber4 containing a fluidized bed 17 of solid particles of relatively purealuminum chloride with an average size distribution of about 48 to below325 mesh (see below) maintained in fluidized condition initially bypassage of gas upwardly through distribution inlet 20.

The bed 17 is cooled by passing water through cooling coils 26 extendingthrough the bed so that the entering gas is rapidly quenched to about 60C. in the relatively pure aluminum chloride particles maintained in thefluidized bed.

Although not fully understood at the present time, the quenched orcooled gaseous aluminum chloride apparently forms solid nuclei particleswhich build up to larger particles and/or deposit on other solidaluminum chloride particles already present in the bed. As the particlesof aluminum chloride increase in size, they are removed constantly fromthe bed via outlet 24, in an average particle size distribution as notedbelow. The average residence time of the aluminum chloride in thecondenser bed is about 2.5 hours.

The oil gas from the fluidized bed 17 is passed through the filterassembly 21 causing the return of entrained aluminum chloride solids anddust directly back to the bed 17.

The aluminum chloride product recovered via outlet 24 is a relativelyfine and flowable highly purified desublimated solid product, which isessentially free from sodium, iron, silicon and titanium impurities, andwhich has less than about 0.3% by weight total content of combined 12oxygen and non-volatile impurities, a low content of adsorbed carbondioxide and phosgene (trace amounts), and average particle sizedistribution of:

Percent +48 mesh (retained) 1 48 to mesh 2 80 to mesh 10 100 to +200mesh 47 325 mesh (passes through) 40 Percent 1 Balance.

What is claimed is:

1. In the production of aluminum chloride by the chlorination of sodiumcontaminated alumina, a process for selectively recovering high purityaluminum chloride from the hot gaseous eflluent thereof containingaluminum chloride values, carbon oxides, aluminum oxychloride values,volatilized sodium aluminum chloride values and entrained particles ofalumina and carbon, comprising the steps of initially cooling said hotgaseous effluent to a first predetermined temperature range below thechlorination reaction temperature and well above the ambient conditioncondensation temperature of aluminum chloride efiective to selectivelycondense a first portion of the sodium aluminum chloride values therein,separating such initially condensed values and entrained particles fromsaid gaseous eflluent, further cooling the residual gaseous efiluenttherefrom to a second and lower predetermined temperature range abovethe ambient condition condensation temperature of aluminum chlorideefiective to condense a high proportion of the remaining volatileconstituents therein that are condensable above the condensationtemperature of aluminum chloride, separating such secondary condensatefrom such gaseous effluent, and directly desubliming high purityaluminum chloride values from such efiiuent in a fluidized bed ofaluminum chloride particles at a third predetermined temperature rangebelow the ambient condition condensation temperature of aluminumchloride.

2. process according to claim 1 wherein the initially separated valuesalso include aluminum oxychloride values.

3. Process according to claim 1 wherein the secondarily separated valuesalso include effectively all of the remaining aluminum oxychloridevalues, together with such high proportion of condensed sodium aluminumchloride values from the further cooled gaseous efiluent.

4. Process according to claim 1 wherein said step of initially coolingsaid hot gaseous efiluent reduces the temperature thereof to betweenabout ZOO-600 C.

5. Process according to claim 1 wherein said step of further cooling theresidual gaseous etlluent reduces the temperature thereof to betweenabout ISO-250 C.

6. Process according to claim 1 wherein said third predeterminedtemperature range is about 30-100 C.

7. Process according to claim 1 wherein said first predeterminedtemperature is between about ZOO-600 C., said second predeterminedtemperature is between about l50-250 C., said third predeterminedtemperature range is between about 30-100 C., and the solidifiedaluminum chloride is recovered in an average particle size distributionof about 40350 mesh.

8. In the production of aluminum chloride by the chlorination of sodiumcontaminated alumina in the presence of a reducing agent, a process forselectively recovering high purity aluminum chloride from the hotgaseous effluent thereof containing aluminum chloride values, carbonoxides, aluminum oxychloride values, volatilized sodium aluminumchloride values and entrained particles of alumina and reducing agent,comprising the steps of initially cooling said hot gaseous efiiuent to afirst predetermined temperature range below the chlorination reactiontemperature and well above the ambient condition condensationtemperature of aluminum chloride eifective to selectively condense ahigh proportion of the sodium aluminum chloride values therein,separating such initially condensed values and entrained particles fromsaid gaseous efiluent, further cooling the residual gaseous efiiuenttherefrom to a second and lower predetermined temperature range abovethe ambient condition condensation temperature of aluminum chlorideelfective to condense a high proportion of the remaining volatileconstituents therein that are condensable above the condensationtemperature of aluminum chloride, separating such secondary condensatefrom such gaseous effluent, and directly desubliming high purityaluminum chloride values from such efiluent in a fluidized bed ofaluminum chloride particles at a third predetermined temperature rangebelow the ambient condition condensation temperature of aluminumchloride.

9. Temperature gradient process for the removal of impurities from a hotgaseous efiluent from the chlorination of sodium contaminated aluminacontaining aluminum chloride, condensable sodium aluminum chloride,carbon oxides, entrained particles of aluminum oxychloride, aluminumhydroxychloride, alumina and carbon, comprising the steps of initiallycooling such hot gaseous efiluent to a first predetermined temperaturebetween about 200- 600 C., to condense a substantial portion of thesodium aluminum chloride values, initially separating such condensedconstituents and entrained particles from said gaseous effluent andreducing the temperature of said gaseous efiluent to a second and lowerpredetermined temperature of between about 15 0-25 0 C. to condenseessentially all of the remaining condensable volatile constituentstherein which are condensable above the condensation temperature ofaluminum chloride, separating the thereby condensed remainingconstituents therein and essentially all of the remaining entrainedparticles from the thereby purified aluminum chloride containing furthercooled gaseous efiluent, and directly desublinring gaseous ahuninumchloride to solid form from such purified further cooled gaseousefiiuent in a fluidized bed of solid particles of aluminum chloridemaintained at a temperature substantially below the ambient conditionsolidification temperature of aluminum chloride.

References Cited UNITED STATES PATENTS 1,566,269 12/ 1925 Burgess 4231352,446,221 8/1948 Ferguson 423-135 X 3,175,883 3/1965 Lindsay et al423-133 3,406,009 10/ 1968 Gould et al --71 X 3,582,262 6/1971 Tomany55-71 EDWARD STERN, Primary Examiner US. Cl. X.R.

